This application is based upon and claims the benefit of priority of the prior Japanese Patent Applications No. 10-126288 filed on May 8, 1998, No. 10-369840 filed on Dec. 25, 1998, and No. 11-41967 filed on Feb. 19, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a capacity type semiconductor dynamic quantity sensor which provides the displacement of a movable portion in response to a dynamic quantity acting thereon as a sensor output and a method for manufacturing the same.
2. Description of the Related Art
For example, a capacity type semiconductor acceleration sensor has a configuration in which a beam structure is provided as a movable portion displaced in response to acceleration acting thereon. The displacement of the beam structure is output as a change in the capacity of a capacitor formed between a movable electrode provided integrally with the beam structure and a fixed electrode provided on a substrate. As disclosed in Japanese Patent Application Laid-Open No. 6-349806 and etc., such a semiconductor acceleration sensor has been manufactured using a method including the steps of providing a semiconductor substrate having an SOI structure by forming a second semiconductor layer on a first semiconductor layer (support substrate) with an insulation layer interposed therebetween, patterning the first semiconductor layer into a predetermined configuration in accordance with the configurations of the beam structure, fixed electrode and the like, and etching the insulation layer as a sacrificial layer. The beam structure having the movable electrode and the fixed electrode is eventually formed on a semiconductor substrate.
The above-described manufacturing method must inevitably include the step of etching the sacrificial layer using an etchant. At the step of etching the sacrificial layer, the surface tension of the etchant frequently causes the so-called sticking phenomenon that is sticking of the beam structure (especially the movable electrode) to other regions (particularly, the fixed electrode). Such a phenomenon leads to a failure in that the detection of any change in the capacity between the movable and fixed electrodes is disabled. This has resulted in a problem with the manufacturing method in the prior art in that it inevitably results in a reduction of yield.
It has been also revealed in that a conventional capacity type semiconductor dynamic quantity sensor has a problem as described below when the second semiconductor layer having the movable and fixed electrodes formed therein and the first semiconductor layer supporting the same (support substrate) have relatively high specific resistances.
Specifically, in general, a capacity type semiconductor dynamic quantity sensor utilizes a carrier wave signal having a relatively high frequency to output a change in the capacity thereof. In this case, when a voltage in accordance with the carrier wave signal is applied between the movable and fixed electrodes forming a capacitor, charge is generated at a side of the movable electrode which corresponds to the product of the capacity of the capacitor and a potential difference between the fixed electrode and itself. Any change in the capacity of the capacitor is output using the movement of the charge.
The charge movement thus caused involves a path for applying the voltage to the movable and fixed electrodes and a time constant that depends on the resistance and capacity of a path for outputting the charge. However, since such paths include resistive elements (including the movable and fixed electrodes themselves) formed by the second semiconductor layer and parasitic resistive elements formed on the first semiconductor layer through the insulation layer, the time constant is larger when the first and second semiconductor layers have relatively high specific values of resistance. Such a large time constant suppresses the rate of the charge movement and delays the rise of the carrier wave signal. Therefore, the detection of dynamic quantities utilizing the capacity of the capacitor provides results deviated from values which should normally be obtained, resulting in deterioration of dynamic quantity detection characteristics.
The present invention has been conceived taking the above-described situation into consideration. It is a first object of the invention to provide a method for manufacturing a semiconductor dynamic quantity sensor which is advantageous in that a movable electrode thereof can be reliably prevented from sticking to other regions during manufacture of the same to allow improved yield. It is a second object of the invention to provide a capacity type semiconductor dynamic quantity sensor in which dynamic quantity detection characteristics can be improved with a simple configuration.
To achieve the first object, according to a method for manufacturing a semiconductor dynamic quantity sensor of the present invention, when a movable portion is formed in a first semiconductor region that is provided on a second semiconductor region with an insulation film interposed therebetween, both the first semiconductor region and the second semiconductor region are etched to form a movable portion in the first semiconductor region. At that time, the movable portion is finally defined at a movable portion defining step that is carried out in a vapor phase atmosphere. In this method, the movable portion does not stick to other regions due to an etchant during the manufacture thereof, resulting in improved yield.
Preferably, the step of forming the movable portion includes steps of forming a trench in the first semiconductor region, etching the second semiconductor region to expose at least a portion of the insulation film corresponding to the trench, and performing the movable portion defining step. More preferably, after forming the trench, a protection film is formed on the first semiconductor region and in the trench. In this case, one of the first semiconductor region, the second semiconductor region, the insulation film, and the protection film is etched at the movable portion defining step to finally define the movable portion. The second semiconductor region may be etched first by a first etching step using an etchant to be a specific thickness and be etched by a second etching step in a vapor phase atmosphere. In this case, the second etching step and the movable portion defining step can be successively performed without changing an etching condition. Otherwise, the semiconductor layer may be etched only in the vapor phase atmosphere, resulting in a simplified process.
Before performing the movable portion defining step, a dicing step may be carried out to cut the semiconductor substrate into a sensor chip so that it can be carried out without damaging the movable portion.
After performing the movable portion defining step, a hydrophobic thin film may be formed on the movable portion. The hydrophobic thin film prevents the movable portion from sticking to other regions due to an electrostatic force not only at the manufacture thereof but at a sensor operational state, and the like. This is because the surface energy of the movable portion is decreased by the hydrophobic thin film so that even when it sticks to an object, it is easily detached from the object. The step of forming the hydrophobic thin film can be carried out simultaneously with the movable portion defining step in the vapor phase atmosphere.
To achieve the second object, a semiconductor dynamic quantity sensor of the present invention includes a semiconductor support substrate having a specific resistance equal to or less than 3 xcexa9xc2x7cm and a semiconductor layer provided on the support substrate with an insulation film interposed therebetween and having a specific resistance equal to or less than 3 xcexa9xc2x7cm. A movable electrode and a fixed electrode are provided in the semiconductor layer to form a capacitor therebetween. The capacitor provides a capacity that changes in response to displacement of the movable electrode to detect the dynamic quantity.
In this dynamic quantity sensor, since the values of the specific resistance of the support substrate and the semiconductor layer are equal to or less than 3 xcexa9xc2x7cm, not only the movable electrode and the fixed electrode but also parasitic resistors provided on the support substrate side have low values of resistance as well. As a result, variation of value detected based on the capacity of the capacitor between the movable and fixed electrodes is decreased, resulting in improvement of detection characteristics.
Preferably, the fixed electrode includes a first fixed electrode portion forming a first capacitor with the movable electrode and a second fixed electrode portion forming a second capacitor with the movable electrode. The first capacitor and the second capacitor provide first and second capacities that change on a differential basis according to the displacement of the movable electrode. Accordingly, a change of the electrostatic capacities are apparently increased relative to the dynamic quantity to be detected to improved an S/N ratio.